This basic picture book offers a great starting place for parents - it deals with the difference between boys and girls, without going into the whole "where babies come from" section.
Mostly, the cartoon illustrations deal with all the body parts we have that are the same: legs, ears, noses, belly buttons, and nipples. Then it deals briefly (and again, with simple cartoon drawings) about the parts we have that are different: dogs have tails, boys have penis, and girls have vagina.
Every little girl with a brother knows this stuff - it isn't information we should hide or be embarrassed about. Yet it's hard if your family isn't gender-mixed to know how these things should be approached. Harris does a nice job of beginning the conversation for you.
Later, when kids want to know about sex ... there are other books and resources. This one's more for the basic, pre-kindergarten discussion.